We live in an era when it is not unusual for exoplanets—worlds beyond our solar system—to be discovered in batches of hundreds. The vast majority of these planets are exposed to astronomers by their host stars, either when the planets transit in front of them, temporarily blocking stellar light, or when they exert an observable gravitational force on their stars.
But there are some worlds that wander the Milky Way as solar exiles, catapulted from their native systems by interloping objects or cataclysmic events. Others are born in the interstellar medium without a parent star. These planets, with no sun of their own, go by many names—rogues, nomads, orphans—and there are estimated to be billions of them adrift in our Milky Way, sparking the imaginations of scientists and science fiction fans alike.
Such free-floating places are shrouded in mystery relative to their star-anchored peers, because they lurk in the shadows of the galaxy, and leave few traces of their presence. But they are not entirely undetectable, as demonstrated by new research published on Monday in Nature.
Scientists led by Przemek Mróz, a PhD student at Warsaw University Observatory, analyzed the light curves of nearly 50 million stars observed between 2010 and 2015 by the Optical Gravitational Lensing Experiment (OGLE) at Las Campanas Observatory in Chile. The team flagged signs of microlensing events, which occur when the gravitational field of an object, such as a rogue planet, distorts and brightens a background light source as it passes in front of it, illustrated in the animation below.
Mróz and his colleagues winnowed down these five years of observation to 2,617 high-quality microlensing events, a significantly larger sample size than the same instrument's previous haul, published in 2011, which compiled 474 events.
There is likely a huge population of Earth-scale nomad worlds in the Milky Way
The duration of these microlensing events correlates to the mass of the object, with lensing of Earths and super-Earths lasting for only a few hours, Jupiter-mass worlds lasting one to two days, and stars lasting several days. Mróz's team statistically analyzed the distribution of these timescales, and found that six of the events were shorter than half a day, indicating they are likely planets that are roughly as massive as Earth. (Some of these suspected objects may be "wide-orbit planets," meaning that they are gravitationally bound to a star, but orbit at such an extreme distance that they appear indistinguishable from rogue objects.)
The new study found that Jupiter-scale rogue planets are far rarer than what had been suggested in the 2011 study, with an estimated upper limit of 25 Jupiter-mass objects per 100 main-sequence (fusion-capable) stars. This is about ten times lower than the 2011 results, Mróz told me over email, adding that the new number "is consistent with our expectations from planet formation theories."
That is great news for anyone who fears unexpected collisions between Earth and unmoored gas giants like Jupiter, the biggest planet in our own solar system. In the highly unlikely event this ever happens, it could set Earth aflame with tidal forces, or simply swallow our world whole.
Mróz's team also found that there is likely a huge population of Earth-scale nomad worlds in the Milky Way, with an estimated two rogue Earths for every main-sequence star. According to Mróz, it will be possible to image more distorted signatures produced by shadow Earths with a new generation of space-based observatories, like WFIRST and Euclid, which are slated for launch over the coming decade.
"We've currently reached the maximum sensitivity to short-timescale events from a single Earth-based observatory," Mróz told me. "This is simply because we are able to continuously monitor the galactic center for maximally ten hours every night. Future space-based missions [...] will be able to observe the galactic bulge continuously, so they will have much higher sensitivity," for capturing ultrashort (hours) and short (1-2 days) events.
These data-rich microlensing surveys are allowing scientists to build a census of rogue planets, and a better understanding of how these worlds become stranded between stars. Improving detection techniques could help us spot any wandering worlds that could potentially mess with Earth, a la Melancholia, though these encounters are improbable. More significantly, the Milky Way's hidden starless planets are a relatively unexplored group of exoplanets, that could shed light on everything from star system evolution to the limits of extraterrestrial life.
"I think that our observations will help to constrain planet formation theories," Mróz said. "For a long time scientists have predicted that some planets may be ejected from parent systems during the planet formation process. Such planets don't emit light, so gravitational microlensing is the only technique that can detect them."
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